Endothelial cells lining the blood vessels of a donor organ are the major target of immunity and an important effector of tissue injury in xenogeneic organ grafts. The overall objective of this project is to elucidate how endothelial cells mediated xenograft rejection. Xenograft rejection is triggered by the reaction of recipient antibodies and complement with donor endothelial cells. This reaction leads very rapidly to thrombosis and a loss of vascular integrity. The first objective of the project will be to discover how the binding of antibodies and the activation of complement might cause changes in the morphology of endothelial cells or the loss of heparan sulfate from the cells which would lead to these pathological changes. The second objective of the project is to study the pathogenesis of acute vascular rejection. The hypothesis is put forward that this type of rejection reflects the activation of graft endothelial cells due to the prolonged interaction with recipient antibodies and complement. This hypothesis will be pursued by studying manifestations of endothelial cell activation in cultured pig endothelial cells mediated by the binding of human antibodies and the activation of complement components. The manifestations to be studied include prothrombotic changes such as he increase in tissue factor and decrease in thrombomodulin, proinflammatory changes such as the synthesis of cytokines and elaboration of cell adhesion molecules and changes in the barrier properties of a cultured monolayer. The third objective of the project is to elucidate the mechanism behind the phenomenon of "accommodation." Accommodation reflects an acquired resistance of endothelial cells to injury caused by antibody binding and complement activation. This change may result from a decrease in the sensitivity of endothelial cells to humoral injury or an increase in the production of protective factors. A better understanding of the mechanisms of endothelial cell injury in xenotransplantation may contribute to the clinical application of xenotransplantation and also toward a further understanding of the mechanisms underlying vascular disease.